Flow measurement is important in many fields. For example, many industrial processes require measurement of flow through various conduits in order to control the process appropriately. Other uses requiring measurement of a liquid or gas include delivery of a product to a consumer, such as gas, oil, and water. In the medical field, liquid measurement is sometimes applied to a patient's urine output.
Most flow measurement systems rely upon various assumptions regarding the properties of the liquid to be measured and will not work or must be adjusted to cope with deviations from the assumed properties. For example, one well-known technique applies thermal transfer principles applying King's Law to determine the flow rate. With this approach, the thermal properties of the liquid to be measured must be known in advance.
Thermal transfer flow meters typically measure flow continuously using a heating element and two temperature sensors (one upstream & one downstream from, or adjacent to, the heater). By measuring the temperature differential between the two thermometers, the flow is calculated. Alternatively, the temperature is kept constant at the heater and the energy required to do so is monitored, from which the flow can be calculated.
FIG. 1 schematically shows the basic arrangement of a prior art thermal mass flow meter. A liquid flows through a tube 100 in a direction indicated by the arrows. At some location in wall of the tube is placed heating element 120 with temperature sensor 110, which measures temperature Ti, and temperature sensor 112, which measures temperature Tj, located respectively upstream and downstream of heater 120. Isothermal lines 130, 131, and 132 symbolically show the temperature distribution as a result of the power input to the heating element, where the T130>T131>T132.
The calculation for determining the flow rate is according to the formula:W=Q/t=ρ·{dot over (V)}·Cp·(Tj−Ti)  equation 1
Solving for {dot over (V)}:
                              V          .                =                  W                      p            ·                          C              p                        ·                          (                                                T                  j                                -                                  T                  i                                            )                                                          equation        ⁢                                  ⁢        2            
And noting thatW=I·v 
And substituting, yields
                              V          .                =                              I            ·            v                                              ρ              ·                              C                p                            ·              Δ                        ⁢                                                  ⁢            T                                              equation        ⁢                                  ⁢        3            wherein the symbols used herein are defined in the following table:
SymbolMeaningUnitsVVolume[l] Liters {dot over (v)}Volumetric Flow (volume/time)      [          l      min        ]    ⁢          ⁢  Liters  ⁢      /    ⁢  minute QEnergy, work[J] JoulesPPower[J/sec] Joules/second ρDensity      [          g      l        ]    ⁢          ⁢  grams  ⁢      /    ⁢  liter CpSpecific Heat Capacity (under constant pressure)      [          J                        g          ·          °                ⁢                                  ⁢                  C          .                      ]    ⁢          ⁢  Joules  ⁢      /    ⁢      (                  gram        ·        °            ⁢                          ⁢              C        .              )   TTemperature[° C.] degrees CelsiusTiTemperature of liquid before the[° C.] degrees Celsiusheater (upstream)TjTemperature of liquid after or at the[° C.] degrees Celsiusheater (downstream)ICurrent[A] AmperesνElectric potential[ν] VoltsΔTTemperature Difference Tj − Ti[° C.] degrees CelsiustTime[s] seconds
A related type of thermal transfer flow meter, known, inter alia, as a constant temperature flow meter, uses a similar arrangement to that shown in FIG. 1 with the exception that temperature sensor 112 is adjacent to, or integral with heating element 120. In this configuration, the heating element 120 is heated to a set constant differential temperature Tj (as measured by sensor 112) above the temperature Ti measured by sensor 110. As the flow varies, the amount of heat carried away by the flow varies. The temperature of heater 120 is kept constant by adjusting the current (assuming constant Voltage) applied thereto. The variation of the current required (I) to maintain a constant temperature differential ΔT provides a means to calculate the flow, as shown in equation 3.
As can be seen above, in order to accurately measure the flow rate using a thermal transfer flow meter, the density and the heat capacity of the measured liquid must be accurately known.
In some applications, there is no a priori knowledge of the liquid's properties, e.g. heat capacity and density. Some liquids can have varying properties—for example, urine is a liquid whose constituent components can vary from person to person, and, for a single person, can vary over time. As another example, milk can have varying fat content. In some applications, such as at a fuel terminal the same pipe may be used to transfer different types of fuel or gas or even sometimes either intentionally or unintentionally mixtures of gas and liquid products. In all of these situations, the readings of conventional thermal flow meters will be inaccurate and to improve the results the flow meters must be recalibrated on the basis of either assumptions that must be made about the properties of the liquid or empirical measurements.
In some cases, urine of bedside patients is measured manually, where urine flows along a catheter to a urine collection container and hospital personnel visually estimate the patient's urine output (ml/h) from the urine collection container. In practice, this arrangement is laborious and inaccurate, since hospital personnel must manually determine the amount of hourly urine and the dynamic nature of critical care settings makes it difficult to adhere to timely measurement. A simple, easy-to-use solution for measuring urine flow is needed to assist in accurate and timely measurement of urine output.
A brief understanding of related prior art can be gained from U.S. Pat. No. 6,536,273, which discloses a thermal flow rate sensor that can be used with liquids of variable composition. The sensor comprises two elements: a conventional thermal flow sensor and a thermal-conductivity measuring cell. The thermal-conductivity measuring cell is used to determine the composition of the liquid and the results of measurements made from this cell are combined with other calibration measurements to correct the measurements made by the flow sensor for the properties of the liquid.
It is therefore an object of the invention to provide simple, cost-effective, and accurate flow rate meters, which enable measuring the flow rate of a liquid without knowing beforehand the (possibly dynamic) physicochemical characteristics of liquid being measured.
It is another object of the invention to provide medical systems comprising the flow rate meters of the invention which enables monitoring the flow rate of a biological liquid from a patient.
It is a further object of the invention to provide a method for determining the flow rate of a liquid without knowing the possibly dynamic physicochemical properties of the tested liquid beforehand.
Further purposes and advantages of this invention will appear as the description proceeds.